1. Field of the Invention
The present invention relates to an ink composition for a water-based ballpoint pen, and specifically to an ink composition for a water-based ballpoint pen in which an oil phase is contained in an aqueous phase in a state of an oil-in-water type emulsion.
2. Description of the Related Art
A ballpoint pen is roughly classified into two types: a water-based ballpoint pen using a water-based ink and an oil-based ballpoint pen using an oil-based ink. Since the water-based ink commonly has low viscosity, a large amount of the ink flows out of the water-based ballpoint pen during writing, and a clear drawn line is obtained, and thus splitting, starving, and a blobbing phenomenon are less likely to occur. The water-based ballpoint pen also writes smoothly because of low viscosity of the ink. However, on the other hand, the water-based ballpoint pen also has a drawback that a drawn line is likely to be feathered and writing unsmoothness is recognized during writing because of a large flow-out amount. To the contrary, the oil-based ballpoint pen causes no feathering of a drawn line and also exhibits less writing unsmoothness during writing because of high ink viscosity compared with the water-based ballpoint pen. However, because of a small flow-out amount during writing and high viscosity, the water-based ballpoint pen does not often write smoothly, and also often causes splitting or starving as well as a “blobbing” phenomenon due to an untransferred ink.
JP 62-501914 W discloses a gel ink ballpoint pen, in which feathering of a drawn line is reduced by imparting thixotropic nature to a water-based ballpoint pen ink, is developed so as to solve the above drawbacks of the water-based ballpoint pen. Although feathering of drawn line has been improved in a current gel ink ballpoint pen, writing unsmoothness has not yet sufficiently improved.
In recent days, there has also been developed an oil-based ballpoint pen in which feel of writing has been improved by decreasing viscosity of an oil-based ink. However, since the flow-out amount during writing increases in the oil-based ink having low viscosity, drawn line-drying property deteriorates, and thus causing strike-through of an ink to the rear surface of a paper, and blobbing.
JP 2004-115611 A discloses an oil-based ballpoint pen ink having low viscosity in which a W/O type organic solvent prepared by mixing an organic solvent with water. However, since water has a problem with storage stability and the ink is mostly composed of an organic solvent, strike-through may occur when the flow-out amount during writing is increased.
JP 2007-327003 A discloses a W/0 type emulsion ink composition in which a pigment is added in an aqueous phase component in advance, followed by mixing with an oil-based component, whereby, water droplets composed of the aqueous phase component comprising the pigment dispersed therein are dispersed in the oil-based component. The publication discloses a ballpoint pen ink in which feel of writing has been improved by using this ink composition. However, the ballpoint pen ink lacks in stability, that is, water droplets are localized as a result of aggregation when stored over the long period. Because of a large amount of an oil-based component in the ink composition, strike-through and drawn line-drying property significantly may become inferior when the flow-out amount during writing is increased, like the water-based ballpoint pen.
The specification of JP 2009-292872 A by the present applicant discloses an ink composition for a water-based ballpoint pen, which has merits of a water-based ballpoint pen and an oil-based ballpoint pen, and also has ink properties in an oil-in-water type emulsion (0/W emulsion) form. This oil-in-water type emulsion ink composition for a water-based ballpoint pen is a new type of an ink composition for a water-based ballpoint pen and has excellent performance capable of achieving very smooth feel of writing. It has been found that, because of insufficient stability during storage at high temperature and storage at low temperature, the ink may cause phase separation when stored over the long period.